Boolean Issue: hole in mesh is.... strange
Thee problem is most likey due to your meshes not being manifold (ie, completely enclosing a single solid volume). The plane is obviously non-manifold (being just a plane) and the blocks are technically non-manifold since they enclose multiple volumes rather than a single connected volume. 

The Boolean modifier includes a 'Solver' setting. This allows you to choose between BMesh and Carve and these use different methods to apply the boolean which can produce different results - especially with non-manifold meshes. 

I can replicate your results using Carve and switching to BMesh produces the expected cut-out.

 

NOTE : The Solver option was introduced after Blender version 2.76b. If you do not have that option then you'll need to address this in a different way - by making the meshes manifold as this results in the modifier producing more consistent results. To make the plane manifold you can simply add a Solidify modifier before the Boolean modifier. With the plane manifold both solvers should produce the desired result.